This invention relates generally to the use of polymer-based chemiresistor sensing elements in microchemical sensors that are capable of detecting small concentrations of vapors emitted by volatile organic compounds (VOC) or other volatile compounds (VC).
Polymer-based vapor absorption type sensors are attractive choices for use in VOC monitoring devices that are used, for example, for environmental monitoring in the vadose zone. Examples of these microchemical sensors include conductometric sensors such as chemiresistors, surface or thickness-shear mode acoustic wave (SAW) mass sensors, flexural plate wave mass sensors, and MEMS microcantilever mass sensors. Chemiresistors are a particularly simple and inexpensive type of chemical sensor whose electrical resistance changes when exposed to certain chemical vapors. Micro-sized chemiresistor sensors are relatively easy to fabricate using well-known semiconductor fabrication techniques; can be made very small (<100 square microns); can operate at ambient temperatures; are passive devices (no pumps or valves are needed); are relatively inexpensive; and their resistance change can be read-out by a simple, low power (and low current) circuit that measures DC resistance. This feature allows the use of long electrical cables, which allows the resistance measurement unit and data logging equipment to be remotely located. Also, chemiresistors are more resistant to chemical poisoning than other types of sensors (e.g., catalytic sensors).
A common type of chemiresistor consists of a chemically sensitive, electrically insulating, organic, soluble polymer matrix that is loaded with a large volume (e.g., 20–40%) of electrically conductive metallic (e.g., gold, silver) or carbon/graphite particles to form a composite material having continuous networks of electrically conductive pathways throughout the polymer matrix (i.e., host). To fabricate a chemiresistor, the polymer is dissolved with a solvent (e.g., water, chlorobenzene, or chloroform) and sub-micron diameter carbon, silver, or gold particles (e.g., 20–30 nanometers) are added to make a “chemiresistive ink” (carbon particles are most commonly used). Typically, 0.1 g of solids (polymer plus carbon particles) are dissolved in 5 mL of solvent. Acoustic vibration using a pulsed, point ultrasonic source can be used to uniformly disperse the particles. A non-ionic surfactant can be added to this mixture to chemically bond to the electrically conducting particles and thereby form steric barriers, which prevent undesirable aggregation or agglomeration of these particles. Spin casting can produce films from 200–400 nm thick, while pipetted films are generally thicker (e.g., 1–10 microns) and less uniform in thickness than the spun films. A filter (e.g., a 5 micron pore size filter) can be used to screen the ink and remove any large agglomerations of the conductive particles prior to deposition. Then, the resulting ink is deposited as a thin film (typically onto an insulating substrate contacting two or more spaced-apart electrodes (i.e. resistor leads), and then dried by evaporation.
When chemical vapors of solvents, toxic chemicals, explosives, or VOCs come into contact with the chemically sensitive polymer composite, the polymer matrix absorbs the vapor(s) and swells. The swelling spreads apart the embedded conductive particles, breaking some of the conductive pathways. This increases the electrical resistance across the two or more electrodes by an amount that is easily measured and recorded. The amount of swelling in steady state, and, hence, the steady-state resistance change, can be uniquely related to the concentration of the chemical vapor(s) in equilibrium with the chemiresistor film. The resistance response is generally linear with increasing vapor concentration, but can become non-linear at high solvent concentrations when the percolation threshold of the polymer composite is reached (i.e., 20–40% swelling). The swelling process is generally reversible; hence the polymer matrix will shrink when the source of chemical vapor is removed (although some hysteresis can occur).
The polymer matrix used in chemiresistors can absorb multiple solvents having similar solubility parameters. Since it is unlikely that any specific polymer will be sensitive to only one particular VOC, an array of multiple chemiresistors containing a variety of polymer hosts is generally needed to provide accurate discrimination among multiple, interfering vapors (including water vapor).
Multiple chemiresistors have been fabricated side-by-side on a common substrate, such as a silicon wafer, where each chemiresistor has a different thin film polymer matrix selected for high sensitivity to a particular VOC of interest. The more unknown VOCs there are, the greater the number of different polymers is needed to provide adequate discrimination. Hence, a fast and accurate identification technique is needed that can distinguish between multiple types of solvents (polar and non-polar), for both pure compounds and mixtures, over a wide range of concentrations, and in the presence of water vapor.
A common and obvious source of interfering vapors is water vapor (i.e., relative humidity) in the ambient environment. Water vapor affects the relative sensitivity of certain polymers to solvent vapors, and affects the patterns of responses obtained from arrays containing those polymers. A microchemical sensor that is capable of identifying the maximum number of possible analytes should have multiple chemiresistor elements that are as chemically varied as possible, with at least one chemiresistor having significant sensitivity to water vapor.
Arrays of chemiresistor sensing elements fabricated on a single substrate have been successfully used to detect a wide variety of VOCs, including aromatic hydrocarbons (e.g., benzene), chlorinated solvents (e.g., trichloroethylene (TCE), carbon tetrachloride, aliphatic hydrocarbons (e.g., hexane, iso-octane), alcohols, and ketones (e.g., acetone)). Other VOCs of interest to groundwater protection include methyl tert-butyl ether (MTBE), other gasoline additives, toluene, and xylene.
Chemiresistor and surface-acoustic wave (SAW) sensors use chemically sensitive polymer composite films to absorb/adsorb analytes of interest, which produces changes in the sensor's characteristics (e.g., resistance, resonance vibration frequency, etc.). A variety of solvent casting techniques have been used to deposit the polymeric coatings on these sensors, including: spray coating, spin casting, dip coating, painting with small brushes or Q-tips, dabbing, and syringe deposition. In solvent casting, the polymer with particulate additive is dissolved/suspended in a solvent to form a solution, which is then deposited on a substrate and allowed to dry, whereby the solvent evaporates to leave the desired coating material.
The repeatability, reliability, and performance of these microchemical sensors depend on the volume, area, electrode contact area, and composition (including uniformity of composition) of the deposited chemically sensitive polymer composite. Unfortunately, these parameters are not well controlled using current methods of solvent casting. Many of the methods described above rely on gravity-driven or pressure-driven means to deposit dissolved polymers onto a solid surface. Tools that are used in these methods require that the solution be placed inside a machine or tool that squirts or drips a microdrop of liquid (e.g., micrograms) onto the desired surface. Precise placement and volume control is challenging. In the fabrication of chemiresistor-type microchemical sensors (where conductive particles are dispersed within a polymer matrix), clogging can occur if the solvent dries while the chemiresistive ink is introduced or stored in the machine, or while the ink is being ejected from the machine. This creates unacceptable variability in the conductive particle/polymer ratio, which directly affects the repeatability and performance of chemiresistors; as well as reducing the production yields (e.g., yields greater than 30% are difficult to achieve due to nozzle clogging with automatic machines). Filters can be used to help reduce clogging, but then the filters get clogged.
Manual techniques, such as hand painting, brushing, dabbing, etc. have been used, but suffer from poor reproducibility and imprecise placement. Micropipettes (i.e., microcaps) have been used to manually deposit microdrops with better volume control and accuracy than by hand brushing. Microcaps having volumes ranging from 1–100 microliters, typically 10–25 microliters, and diameters less than 1 millimeter have been used to deposit drops having a volume less then 1 microliter. However, the accuracy of placement and control of amount of material deposited, is highly dependent on the skill and experience of the technician. Hence, it is not uncommon for the baseline electrical resistance of these manually placed polymer composite films to vary by factors of 2–3, which is clearly unacceptable for commercial manufacture.
FIG. 1A shows a photomicrograph of an example of a microchemical sensor with four chemiresistors, using a linear electrode pattern and four different linear polymer films. The four different polymer films are poly(n-vinyl pyrrolidone) PNVP; poly(vinyl alcohol) PVA; poly(ethylene-vinyl acetate) PEVA; and poly(isobutylene) PIB, and a linear electrode pattern. The microsensor chip is packaged in a 16-pin dual-inline-package (DIP) with outer dimension of about 3 cm by 0.7 cm. In this example, an Asymtek Century Series C-702 Automated Dispensing Unit was used to deposit the conductive inks, with a 27 gauge, ½″ long needle and the 740V Low Viscosity attachment, at a rate of 1.5 inches per second, resulting in a linear polymer film approximately 500 microns wide by 2300 microns long. The samples were dried at room temperature under ambient conditions.
The electrode patterns (i.e., resistor leads) for the four chemiresistors, the two heater strips, and the temperature sensor were made of platinum and titanium created using standard photolithographic techniques on a silicon wafer with a 200 nm thick insulating silicon nitride layer. The platinum (1000 A) on titanium (200 A) features were deposited by evaporation. The electrode patterns comprise four parallel conductors (i.e., metallized traces). This electrode layout provides a four-point resistance measurement, which helps to minimize contact resistance effects. The final dimensions of the deposited film (e.g., thickness, width, length, uniformity etc.) are highly dependent on the deposition rate, wetting characteristics, and viscosity of each ink, among other factors.
FIG. 1B shows a photomicrograph of another example of a microchemical sensor with four chemiresistors, using a linear electrode pattern and four different linear polymer films. In both FIGS. 1A and 1B, the edges and composition of the four different polymer composite films are non-uniform and irregular, despite being deposited by the Asymtek Automated Dispensing Unit. Also, some of the deposited films have spread wider, or more irregularly, than others (e.g., PEVA), likely due to differences in the wetting and drying characteristics of the solvent; combined with the observation that the liquid inks were deposited on a flat substrate with no raised borders or surface features to confine the perimeter of the blob/drop of ink to any particular shape or geometry.
FIGS. 2A and 2B show photomicrographs of other examples of a microchemical sensor with four chemiresistors, using a linear electrode pattern and four different circular polymer films. Here, the polymer composite films were deposited manually as drops using a micropipette technique. The PECH and PIB polymers were dissolved in chloroform; the PNVP was dissolved in water, and the PEVA was dissolved in TCE. However, despite the use of a micropipette tool to deposit nominally “circular” films having a diameter of approximately 1.5–2.5 mm, the edges and composition of the deposited films are also irregular and non-uniform. In some of the dried films, a dark ring can be seen around the outer perimeter of the drop. This is likely caused by a higher concentration of the black carbon particles that have migrated to the outside during deposition and drying, due to the “coffee stain” effect.
Additionally, a large amount of excess material has been deposited on either side of the four parallel electrodes, which causes an unnecessary waste of material. Also, the overall pattern of electrode traces on the substrate (i.e., silicon die) is not particularly compact; since about 50% of the electrode's length is not covered by the chemically sensitive film. Also, the two polymer films on the outside are located closer to the heating element, and might be hotter than the two inside films that are located farther away.
Good adhesion of the polymer composite film to the substrate and the electrodes covered by the film is necessary to make a chemiresistor with good reliability and reproducibility, as well as to minimize batch-to-batch variations and to maximize manufacturing yield. Any partial detachment of the film from the electrodes can create an artificial rise in the resistance reading (i.e., by increasing the contact resistance), which could be incorrectly interpreted as an exposure to VOC vapors (i.e., false positive).
The surfaces of the polymer film in contact with the two or more electrodes are defined as the “electrode contact area”. The ratio of the polymer film's area divided by the total electrode contact area is defined as the “contact area ratio”, which can range from 0 (no contact) to 1 (100% contact). In FIG. 1, and more so in FIG. 2, it can be seen that a large fraction of the polymer film is not in contact with the electrodes. The contact area ratios for the chemiresistors shown in FIG. 2 range from about 15–30%. The lower the contact area ratio is, the greater the sensitivity of the chemiresistor will be to errors caused by any detachment of the polymer matrix from the electrodes. A design that maximizes the contact area ratio will minimize any errors caused by film detachment, thereby improving the stability and reliability of the chemiresistor. A design having a large contact area ratio will also be less sensitive to errors caused by a non-uniform distribution of the conductive particles embedded in the polymer matrix.
Given a fixed area of polymer film, an electrode layout having a larger contact area ratio will have a lower electrical resistance than a design having a smaller contact area ratio.
U.S. Pat. No. 5,951,846 to Lewis and Freund, Sensor Arrays for Detecting Analytes in Fluids, illustrates an interdigitated array of linear electrodes covered by a conducting polymer film having an ideal, perfectly rectangular shape (See Lewis and Freund, FIG. 1A-1 and FIG. 4A-1). Lewis and Freund, however, do not discuss the practical problems of obtaining reproducible and reliable performance, with high manufacturing yields, when depositing realistic polymer composite films having less-than-ideal shapes, thickness variations, and non-uniform composition, etc.
The distribution of conductive particles (e.g., carbon) within a polymer film can be non-uniform if the particles have not been well mixed, or if large clumps or colloids of particles form during deposition and drying. FIG. 3A shows a plan view of a first example of a chemiresistor comprising a circular polymer composite film contacting a pair of linear electrodes, with clusters of agglomerated carbon particles non-uniformly distributed near the center of the film. FIG. 3B shows a plan view of a second example of a chemiresistor comprising a circular polymer composite film contacting a pair of linear electrodes, with clusters of agglomerated carbon particles distributed non-uniformly around the outer perimeter of the film.
Even though the chemiresistor films in FIGS. 3A and 3B have approximately the same number of carbon particle clusters, the distribution of the clusters is quite different. Since the linear electrode pattern contacts only a small area near the center of the film, it is clear that the total resistance of the chemiresistor in FIG. 3A will be quite different (i.e., lower) than that of FIG. 3B, due to the vastly different distributions of clustered carbon particles.
Some solutions to this problem are to screen out the large clusters of carbon particles before deposition using a filter, or to prevent their agglomeration by using ultrasonic vibration to sonify and disperse the particles during deposition and/or drying, or to coat the particles with a anti-steric material that inhibits agglomeration. Another approach is the increase the total contact area of the electrodes in contact with the polymer film, so as to minimize any deviations in the total resistance due to non-uniform distributions of the conductive particles. In this way the electrodes sample a larger volume of the film and effectively smear out any non-uniform distributions.
Never the less, a need remains for improved deposition methods and novel designs for chemiresistor type microchemical sensors that have better repeatability, reliability and performance, in a more compact footprint, and with reduced manufacturing costs.
Against this background, the present invention was developed.